Kugler et al U.S. Pat. Nos. 3,829,374 and 3,941,899 disclose that an electrode composed of a carbon body having a coating of alumina particles fused thereto possesses improved durability. The fused and re-solidified coating (typically 0.1 to 1 mm thick) protects the surface of the carbon body projecting out of the melt from attack by oxidation agents and thus extends the life of the electrode when it is exposed to the action of oxygen at high temperature, as when it is used as an anode in the production of aluminum by fusion electrolysis. In this process (the Hall process) alumina is electrolyzed at temperatures in the range of 950.degree. C. to 1000.degree. C., and in this range the oxygen evolved from the fused salt mixture, the oxygen in the atmosphere and other oxidation agents react rapidly with the carbon in the electrode.
While the parts of these electrodes projecting out of the melt disclosed in these patents possess greatly improved resistance to attack by high temperature oxidation agents, experience has shown that oxidation agents slowly pass through the aforesaid coating and slowly consume the carbon. Complete protection has not been achieved by increasing the thickness of the coating, because, as has now been found, the coating is not uniformly fused to the carbon but contains discontinuities through which oxygen passes, causing a destruction effect at the high temperature at which the electrodes are used.